230 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



The slow neutron reactor has some advantages over the fast neutron 

 reactor, but it suffers also from severe limitations which are not present 

 in the fast reactor. The large mass and surface of the uranium allows 

 very large quantities of heat to be extracted, so that the total power 

 output of a large reactor can be of the order of 1 million kilowatts 

 of heat. However, it is not yet practicable to remove this heat at a 

 temperature high enough for efficient heat engines to be operated from 

 the steam or gas which carries away the energy. This is due to the 

 limited corrosion resistance of aluminum. The development of coat- 

 ings of beryllium, or the use of less reactive gases such as helium, for 

 cooling, would permit the production of useful power, but no reactor 

 is yet operating under these conditions. There are also other grave 

 difficulties due to the changes in the properties of materials under 

 the action of fast neutrons. These technological problems will be 

 solved, but they take time and great effort to achieve a completely 

 satisfactory answer. A far more serious problem is that in a slow • 

 neutron reactor it is possible to use only a small fraction of the U^*^ 

 present in the rods. Inevitable impurities in the materials and the 

 accumulated products of fission "poison" the reaction by absorbing 

 neutrons. The former set a lower limit to the concentration of U^^ 

 in the rods at which the reactor will operate, and this is only slightly 

 less than the concentration in natural uranium. The uranium must 

 be removed periodically and be subjected to an elaborate and costly 

 chemical process involving solution of the rods in acid, chemical 

 purification, and reduction to metal again. The whole of the initial 

 chemical operations must be carried out by remote control in concrete 

 or lead enclosures which absorb the harmful radiations coming from 

 the highly radioactive fission products. These fission products, equiv- 

 alent in activity to many tons of radium, must be disposed of in some 

 way, and this is one of the most difficult problems of all. If they are 

 thrown down a disused mine or buried they may reappear in the under- 

 ground water supply with disastrous results. The uncertainty of 

 ocean currents renders it hazardous to dump them into the sea, even 

 into the deep sea when sealed in containers. The radioactivity decays 

 away in time, some of it relatively quickly, some much more slowly, 

 so that if the fission products are stored in vats underground they 

 will, in the course of a generation or two, become harmless. However, 

 the storage capacity required to deal with the radioactive waste prod- 

 ucts of a large reactor is so huge that provision of the necessary under- 

 ground vats becomes an immense undertaking. 



All these difficulties are very real and must not be underestimated. 

 However, they are technological difficulties with which modern applied 

 science is accustomed to deal, and are none of them insuperable. 

 Their solution is certain if the necessary effort is made, and many of 



